1. Field of the Invention
This invention relates to methods and apparatus for optical pulse compression.
This invention relates particularly to methods and apparatus for compressing a relatively long width optical pulse of the kind which is transform limited to a shorter pulse by a compressor which incorporates a novel grazing incident delay line and which is compact and which reduces the ellipticity of the output beam and which provides high efficiency of light transmission through the compressor.
The present invention has particular application for compressing long width laser output pulses haivng pulse widths in the order of 100 picoseconds and for achieving pulse compression ratios as high as 45 to 1 by means of a dispersive delay line which has a grating spacing at a convenient distance of 10-30 centimeters (so that the compressor unit is compact) and which operates at relatively high throughput efficiencies in the range of 35 to 40 percent.
2. Description of the Prior Art
In 1968, Treacy [E. B. Treacy, "Compression of Picosecond Light Pulses," Phys. Lett. 28A, 34 (1968)] reported experiments where he compressed chirped optical pulses with a grating pair. Duguay and Hanson [M. A. Duguay and J. W. Hansen, "Compression of Pulses from Mode-locked HeNe Laser," Appl. Phys. Lett. 14, 14 (1969).] compressed 500 ps mode-locked pulses from a HeNe laser by a factor of 2 using a phase modulator and a Gires-Tournois interferometer. McMullen [J. D. McMullen, "Analysis of Compression of Frequency Chirped Optical Pulses by a Strongly Dispersive Grating Pair," Appl. Opt. 18, 737 (1979)] proposed the compression of 10 to 100 nanosecond pulses with a pair of gratings separated by 100 meters. More recently, Nikolaus and Grischkowsky [B. Nicolaus and D. Grischkowsky, "12.times. Pulse Compressin Using Optical Fibers," Appl. Phys. Lett. 43, 228 (1983) and B. Nikolaus and D. Grischkowsky, "90-fs Tunable Optical Pulses Obtained by Two-State Pulse Compression," Appl. Phys. Lett. 43, 228 (1983)] used the effect of self phase modulation in a single mode optical fiber to chirp an optical pulse, and then compressed the pulse using a grating pair. Johnson et al [A. M. Johnson, R. H. Stolen and W. M. Simpson, "80.times. Single-Stage Compression of Frequency Doubled Nd:YAG Laser Pulses," Appl. Phys. Lett. 44, 729 (1984)] have used this technique with a grating separation of 7 meters to compress the 33 ps pulses from a frequency-doubled, mode-locked CW Nd:YAG laser by a factor of 80. Tomlinson et al [W. J. Tomlinson, R. H. Stolen, and C. V. Shank, "Compression of Optical Pulses Chirped by Self-Phase Modulation in Fibers," Optical Society of America Vol. 1, No. 2,139 (April 1984)] have calculated the grating spacing for compression of 60 picosecond pulses to be between 12 meters and 37 meters. Tomlinson et al have proposed several techniques for reducing the physical space required for grating separation. These techniques include the use of finer groove spacings, folding theo ptical path, and using the compressor in double pass. Dupuy and Bado [C. G. Dupuy and P. Bado, "Five Times Compression of Mode-Locked Argon-Ion Laser Pulses," Taper TuE2, CLEO 1984] have compressed 100 ps pulses using a 1.5 meter grating separation.
All of these publications are incorporated by reference in this patent application in accordance with .sctn.608.01(p) of the Manual of Patent Examining Procedure of the U.S. Patent and Trademark Office.
In the prior art, as noted above, the compression of very long pulses has presented a problem because it has been generally thought that the compression of very long pulses required very big separations between gratings of a grating pair used for producing compression of the very long pulse.
Another problem of the prior art has been choosing a solution for the gratings which enables recovery of most of the light refracted to the correct order so that an unacceptable amount of light is not thrown away in the process.